As is known, many electrical or electronic appliances, such as for example television sets, radios, and hi-fi systems, envisage a low-consumption mode of operation, referred to as “stand-by mode”. In this mode, the electrical appliance is inactive as regards normal operation (for example, display of images for a television set, sound reproduction for hi-fi equipment, etc.) but can be controlled in switching-on through a remote control. As is generally known, an electrical appliance in stand-by mode is in any case supplied through the electric-supply mains, such as domestic power, or battery and consumes energy. The energy consumption is due to the presence of a microcontroller and a sensor connected to the microcontroller, configured for receiving and processing possible commands issued by remote control and supplied for this purpose. Considerable efforts have been made in the last few years to limit current consumption in stand-by mode of electrical appliances, which, so far, generally have levels of consumption of a few watts. However, it is evident that, if the consumption in stand-by mode of a plurality of electrical appliances generally present in dwellings is considered, non-negligible daily consumption levels may be reached.
FIG. 1 shows by means of a block diagram a portion of an electrical appliance 1 (in what follows the portion being referred to as a whole as electrical appliance 1) comprising a power supply circuit 4 (more in particular, a switch-mode power supply circuit SMPS) designed to guarantee operation in stand-by mode of a microcontroller 5 and of a command sensor 6 connected to the microcontroller 5 of the electrical appliance 1. The electrical appliance 1 comprises a supply port 2, which is connected, for example, to the supply mains or to a battery (not illustrated) and receives at input a supply voltage VAL. The supply voltage VAL is hence supplied in input to the power supply circuit 4, which supplies the microcontroller 5 both during the normal operating mode and in stand-by mode. In particular, in stand-by mode the microcontroller 5 should be switched on and be able to process possible commands (for example, the command for switching on the electrical appliance 1) issued via a remote control 7 and detected by the command sensor 6. The electrical appliance 1 moreover comprises a supply switch 8, arranged between the supply port 2 and the power supply circuit 4, configured so as to be operated in conduction or interdiction. The switch 8 may, for example, be a main switch of the electrical appliance 1. If the supply switch 8 is operated in conduction (i.e., it is closed), the power supply circuit 4 and the microcontroller 5 are supplied during the stand-by mode; instead, if the supply switch 8 is operated in interdiction (i.e., it is open), the power supply circuit 4 and the microcontroller 5 are not supplied, and the stand-by mode cannot be activated. In the latter case, the electrical appliance 1 is effectively turned off and cannot be switched on via the remote control 7.
FIG. 2 shows a possible embodiment, of a known type, of the power supply circuit 4. In particular, the power supply circuit 4 is of a flyback type.
If the power supply circuit 4 is supplied by means of an AC supply voltage VAL, it is advisable to connect a rectifier 9, for example a diode rectifier bridge and a filter capacitor, cascaded to the supply port 2, in order to generate in use a DC working voltage V1.
The DC working voltage V1 is then supplied in input to a primary winding 12 of a transformer 11. The primary winding 12 comprises a first terminal 12′ connected to the rectifier 9 and a second terminal 12″. The second terminal 12″ is connected in series to a drain terminal D of a switching transistor 15, for example a MOSFET device, which is in turn connected, through its own source terminal S, to a ground reference voltage GND. Furthermore, the second terminal 12″ of the primary winding 12 is connected in series to a drain terminal D of a turn-on transistor 16, being, for instance, a MOSFET device. The turn-on transistor 16 is connected, via an own source terminal S, to a turn-on capacitor 18, which is in turn connected to a ground reference voltage GND.
The switching transistor 15 and the turn-on transistor 16 are controlled in conduction and interdiction by a driving circuit 19. The driving circuit 19 is moreover connected, through a supply port thereof, to the turn-on capacitor 18, from which it receives the supply during its turning-on step. The supply port of the driving circuit 19 is moreover connected, via a rectifier diode 22, to an auxiliary winding 21 of the transformer 11, which supplies the driving circuit 19 during use, after the turning-on step. Furthermore, a turn-on resistor 23 may be present, connected between a gate terminal G of the turn-on transistor 16 and the second terminal 12″ of the primary winding 12.
Finally, the transformer 11 comprises a secondary winding 24 for generating on an output port of the power supply circuit 4 an output voltage VOUT that supplies the microcontroller 5.
In the operating condition in which the electrical appliance is turned off (the supply switch 8 is open), the turn-on capacitor 18 is discharged and the driving circuit 19 is turned off. Closing of the supply switch 8 does not cause immediate turning-on of the driving circuit 19, but generates a passage of current from the supply port 2 through the primary winding 12 and through the turn-on transistor 16, which in turn charges the turn-on capacitor 18. The turn-on transistor 16 is driven in conduction by means of the turning-on resistor 23, which develops, after closing of the supply switch 8, the biasing necessary for switching on (conduction state) the turn-on transistor 16.
When the voltage on the turn-on capacitor 18 reaches a value VC sufficient to supply the driving circuit 19, the driving circuit 19 turns on and drives the turn-on transistor 16 in interdiction and the switching transistor 15 in conduction. The driving circuit 19 is hence supplied by the auxiliary winding 21.
The turn-on transistor 16 and the turn-on resistor 23 form a turn-on circuit 29 of an active type, operated in order to pre-charge the turn-on capacitor 18 for turning on the driving circuit 19. Following upon closing of the supply switch 8, the electrical appliance 1 can switch to a normal operating mode or to a stand-by mode, awaiting a command (for instance, via the remote control 7) by a user.
Both during the normal mode of use and in the stand-by mode, the switching transistor 15 is operated by the driving circuit 19, for example via a square-wave modulation (pulse-width modulation—PWM) signal, with a frequency usually higher than 16 kHz, and enables to transfer the supply needed for operation of the microcontroller 5 to the secondary winding 24. Consequently, also in stand-by mode the driving circuit 19 is constantly supplied in order to drive the switching transistor 15 appropriately for supply of the microcontroller 5.
Hence, it is evident that the stand-by mode generates an energy consumption that is constant and significant over time on account of the need for supply of the driving circuit 19 and the microcontroller 5.
A possible solution for eliminating the energy consumption in stand-by mode consists in turning off the electrical appliance 1 via the main supply key 8 (however, not always present) or removing the supply physically from the electric-supply mains. These solutions, however, entail the loss of the convenience and practicality of having a complete control of the electrical appliance via remote control.